SDL string clause should treat quoted specifiers as being part of the string

[TzuChieh]

• [ ] "@" should be treated as part of the string if quoted, but things like [string visualizer "@visualizer"] will remove "@" currently
• [ ] Fix the fractal.p2 example scene
• [ ] Render sessions in example scenes should use names directly, without including specifiers
• [ ] Render sessions in render test scenes should use names directly, without including specifiers
• [ ] Detect invalid ESdlClauseValue in TSdlString in debug mode